Why is Eevee giving low res animation output?
Set your video quality to high or lossless. A Bloom effect with dark and redbackground is really bad for image compressing method. Which tend to output a bad resolution or the image if the size of the file did not change.



If you want to make video like a professional artist, you might want to check out how image are stored in each format like jpeg, png, bmp, gif, h.264, h.265 and so on. (Notice that I did not mention mkv and mpeg(mp4) because they are container which work as a directory.)

And you might want to save your video in "Image" format to preserve the best quality while editing video sequence.